Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

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2dbit

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Well, the story I was told is that this motor was out about 1/2 mile from start point running WOT when it slowed and died. Since that moment the motor was hard to pull through.

The carb tear down revealed a piece of debris made it past the filters and clogged the main jet. Tore down the carb, dipped it, rebuilt and tested.

The motor is hard to pull through when cold, but once started any consequent starts are easy to pull. Idle is rough, acceleration is smooth and strong and high end is strong.

I've got 100psi compression on top and 120 on bottom.

The one thing that nagged me was that every time this motor cooled down it was hard to pull start. So, I decided to remove the exhaust manifold and see what I could of the piston/cylinder condition... Not good. I included pics, I'll probably end up tearing down this motor to find out all that's wrong and fix what I can (provided it's not too ridiculously expensive.

So, the point of this thread? Speculation.

First, what do you guys think the real story is, how bad do you guys think the damage is going to be, and how much will it cost for the DIY to complete?

Exhaust Manifold Removed.jpgLower Cylinder.jpgUpper Cylinder.jpg
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

Well, got the motor torn down. Damage is pretty nasty, but could tell that just by looking through the exhaust ports. Here are some more pictures, I am a little confused regarding how this failure occurred, ALL the damage is to the top piston, and primarily on the exhaust side. If it were a lack of lubrication, wouldn't there be damage to the bottom piston as well? I understand that gravity keeps it a little more well fed, but by that much? If it was water that damaged it, I couldn't find any cracks between the water jacket and the cylinder... What do you guys think?

I did find that the seal behind the reeds was displaced. This may explain the rough idle, with occasional misfire...

IMG_4983.jpgIMG_4993.jpgIMG_4992.jpgIMG_4991.jpg
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

I have a feeling finding parts for this motor is going to be a chore... and based on how bad that piston looks, I think at LEAST the top rod bearings should all be replaced...if not all bearings throughout the motor...
 
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matth121

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

not sure why no ones offered an opinion, but looks like your 18 is the same as my supposed 25(you'll have to read my posts) but ive been using this for parts
http://www.ishopmarine.com/ishop/js...cCatFilter=MM&eCatSupFldr=supplier-050-025-MM

should give you most of the parts you need- if youve got it torn down and need to replace a few bearings or seals, do them all- hate to have to do this again next year when you could be boating
Matt
 
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Maxz695

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

check ebay for oversized pistons that include wrist pins and needle bearings. There about $80 each and then you need to have the block bored. Full gaskets and any other item damaged
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

Thank you both for your input, I was a little surprised myself that I've had 100 views and no input! I really want to do this right, and to do it right will require all new bearings and oversizing if needed (which based on this damage is pretty much a sure-thing) The problem with ebay has been that a lot of people post their used garbage. I've seen people go so far as bead blasting all surfaces of a piston then posting it on ebay...
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

oh, and btw: matth121... This motor IS the 25hp. If you do a parts comparison (yes, I took the time to go p/n by p/n) the 18 and 25 are IDENTICAL. The one and only part that sets them apart? Carburetor. Not even the whole carburetor either. Part by part comparison of the two carbs show that the ONLY difference is main jet size and carb body. Everything that mounts to the carb body are the same p/n. So, there ya go. Throw a larger carb throat and more fuel to match and you gain 7 horsepower...
 
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matth121

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

just actually found out that what i thought was a 25 thanks to it's cowling and the fact that theyre identical under the cover, is actually an 18 as well- serial number search at mercurypartsexpress.com shows my exact motor with the proper coils for it( ones I ordered from ishopmarine were for a newer XD style and were held in place by a plastic coil cover, mine bolt to the block with a stud down thru the center. I'm guessing mine looks like yours inside since I have a difference in compression between cylinders too. Let me know how you make out.
Matt
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

18/25...doesn't really matter. This motor, regardless of the completely trashed upper piston still beat the pants off a brand new Mercury 15, identical boats, and with 3 adults in the 18, vs 2 adults in the 15... But that's getting off topic...
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

I am leery about where I should send this block to have it bored/honed. I believe at best locally I might find a motorcycle shop... Does anyone know a good reputable machine shop that can do this work properly for a reasonable price?
It's already going to be ~400 for a powerhead kit (if the place I found actually makes one that fits this motor). At the start of this project seeing that you can get a running 18/25 on craigslist for about $500, I didn't want to spend much more than that.
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

Ouch, got a reply to an email I sent to Mastertech Marine about buying a powerhead rebuild kit...this is what I got in response:
We don't(sell a powerhead rebuild kit), and no one else will either. The pistons are discontinued and not available from anyone I can find. If you can find some used pistons that will work we might be able to get some of the other parts.
Standard rings seem to be available but only in sets of 12 rings for $206.00
 
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matth121

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

ouch is right. damn. keep us posted cause im sure i'm in the same boat as you as far as needing a full rebuild. Check with Laddies on this site- hes helped me immensely, retired master merc mechanic. might be able to steer you in the right direction. Pm him, ask him to read your thread
good luck
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

Thanks for the reference matth121, I sent him a PM. I didn't include any real questions since I hope he'll come share his knowledge and experience in the thread so we can both use it...

Thanks Maxz695 for the link to the rods and crank, I REALLY hope I don't have to go that far!!! (though $75 isn't half bad for two rods and a crank).

I've come to a point where I'm starting to wonder how much has to be replaced on this motor. Initially I came to this wanting to get it done RIGHT. That to me, means replacing the damaged piston, bore/hone the affected cylinder oversize if required, and all new gaskets, seals, and bearings. It's getting bad enough that I'm wondering if this thing is going to have to settle on a used <shudder> piston, and simply honing the affected cylinder to establish crosshatch even if it means some scoring remains. I haven't been able to find a complete rebuild kit (though I am still searching) which means buying all the components individually. That could end up making it a cost prohibitive rebuild, and may lead to only the rod bearings being replaced and possible reuse of the good rod's bearings and crank bearings.
 
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Laddies

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

That engine is not reboreable, you can clean aluminum off the cyl. walls with muratic acid, don't try to hone it off you will damage the plateing, after it's remove you can use a ball hone lightly. If the damage is to great US Chrome can replate 1 or both cyls. that piston is quite bad as it's really bad on the skirt so it should be replaced there are many donor engines out there so check around for used parts.
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

So this is actually a chrome plated cylinder engine... I have been back and forth with what kind of cylinders they are... Seems like the most well traveled part of the bore is shiny like chrome, but the less traveled parts were dull like etched aluminum. I see that in the cylinder there are some aluminum deposits, but I have been tentative about considering muratic acid as I don't want to cause more harm than good...
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

The first two pictures are of the damaged cylinder, each side...the second two pictures are of the non-damaged cylinder, each side...

IMG_5024.jpgIMG_5025.jpgIMG_5026.jpgIMG_5027.jpg
 
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Maxz695

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

Laddies knows best on this one I would take his advice and have them recrhromed
 
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matth121

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

Laddies pointed this site out to me

http://store.eastcoastmarineservice.com/SearchResults.asp?Search=9104A+3

that link is for what i think would be your piston assembly- new
they seem to have alot of parts that are NLA elsewhere, might be a good ressource for ya
find a PN from another site and search it on theres- everything for my carb is NLA, but they have everything, including jets if you want to make your 18 a 25. Good luck

oh, heres a link for your rings- looks like 1 per pkg
http://store.eastcoastmarineservice.com/product_p/39-12810a12.htm

be sure to double check these are the right ones, i'm not 100% sure, but i think thats them
 
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2dbit

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Re: Rebuild Speculation! 1983 18Hp Mercury carb clogged at WOT

Well. Just cleaned and measured the bad cylinder. It's definitely in spec. I've got a longitudinal taper of 0.0000" and a lateral taper of 0.0015" Out of round at the top of the cylinder is 0.0015", middle @ 0.0005" and bottom @ 0.0000". This thing is dang near brand new (not surprising since it's used 1-2 weeks a year). I am really just getting hung up on how critical it is that this cylinder gets the plating redone. @ ~$200 for replating for the one cylinder, it almost seems cost prohibitive considering the expensive involved with replacing all the seals/gaskets, bearings, and piston... all for an engine that sees 1-2 weeks of use per year.
 
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